To provide high resolution imagery of distant extended scenes, relatively large aperture optical systems are required. The aperture diameters are typically on the order of several meters. Increasing the diameter of a conventional telescope increases its resolution of fine detail in proportion to the telescope diameter. The light-gathering power of a telescope also increases as the square of the diameter. It is also true, however, that the cost and difficulty to manufacture an optical telescope increases significantly as the diameter of the telescope is increased. Recent advancements in space astronomy have resulted in the launch and deployment of optical telescopes in space which are able to provide imagery of distant celestial objects having significantly greater spatial resolution and point source sensitivity over imagery obtained from comparable Earth-based telescopes by virtue of simply placing the telescope above the Earth's atmosphere.
An example of the current state of the art in space-based telescopes is the Hubble Space Telescope designated generally by the reference numeral 10 in FIG. 1. The Hubble Space Telescope 10 is designed to be launched and deployed from the cargo bay of the Space Shuttle. The collection surface 12 of the Hubble Space Telescope 10 consists of a single (fill) aperture lens on the order of about 2.5 meters. Since the Hubble Space Telescope completely fills the Space Shuttle cargo bay, the aperture diameter for the telescope is already at a maximum. Accordingly, the resolution limit of the single aperture space imaging system like the Hubble Space Telescope is already at its practical limit.
In order to change the line of site of a single full aperture lens optical system like the Hubble Space Telescope, it is necessary to move the entire telescope support structure and lens to aim the lens to the desired field of view. As is readily appreciated by those of skill in the art, the rigid telescope support structure must be sufficiently stiff such that reaction forces exerted on the support structure during positioning changes do not adversely effect the sensitive image collecting optics of the telescope. The measure of stiffness required translates into added weight and cost. For space applications, it is desirable to reduce weight when ever possible.
It is also known to fabricate certain large optical systems, such as mirrors, as a number of segmented and foldable components in order to reduce fabrication costs and weight, as well as provide a means to package the large mirrors into the cargo bays of existing space launch vehicles. Thus, it is conceivable to build a multi-meter diameter segmented full aperture optical system that is capable of gathering more light than the current state of the art single aperture space telescopes for improved high resolution imagery. However, in order to place a multi-meter diameter segmented full aperture system in space, stiff foldable support structures would be needed in order to compactly stow the multi-segmented fall aperture optical system within the cargo space of a launch vehicle. If thin deformable mirrors are used to save weight, then complex and potentially high bandwidth adaptive optics will be necessary. Further, if the optical system is to be implemented as a phased array, then complex piston and pupil matching control is required. In view of the above factors, a multi-meter diameter segmented full aperture optical system would be relatively heavy and have high technical risk.
It is known to increase image resolution of an optical system merely by increasing the is extent of the aperture without requiring a continuous collecting surface. In this way a number of subapertures could be used in place of a single large aperture to achieve increased resolution.
Earth-based multiple aperture optical systems of the type which include a number of smaller subaperture telescopes capable of providing image resolution of a single large aperture telescope are known from the prior art. Such known earth-based systems (sometimes also referred to as "sparse aperture" or "thinned aperture" optical systems) are limited in their performance by the presence of optical wavefront errors. Adaptive optics are capable of removing these wavefront errors but only if an accurate measurement of the wavefront is available. Therefore, a wavefront sensor must be incorporated into the imaging system. The standard technique for measuring the wavefront errors in earth-based sparse-aperture systems is to measure the interference between different telescope images in order to determine whether or not the subaperture telescopes are in phase. Such interference-type sensors can only make measurements from point sources, such as a star or a laser beacon. Thus, the known multiple aperture optical systems are not able to maintain phasing of the subaperture telescopes for applications which involve complex, extended scenes that do not contain localized point-like objects.
Phase diversity is a known technique that is used to estimate the wavefront errors directly from the image data, irrespective of whether or not the image scene contains point sources. In accordance with the known phase diversity technique, two or more phase-diverse images are collected. One of the images is the conventional focal plane image of the object scene that has been degraded by the unknown wavefront errors. Additional images of the same object scene are formed by introducing a known aberration, for example a defocus error, in the image. This can be accomplished with relatively simple optical hardware. The two images can then be compared to determine the wavefront errors in the imaging system.
To the inventors' knowledge, the sparse or multiple aperture concept for synthesizing a multi-meter full aperture telescope using phase diversity techniques has yet to be successfully implemented. Accordingly, a multi-aperture optical imaging system adapted for compact stowage within and deployment from existing satellite launch vehicles and which can provide image resolution equal to or better than current state of the art full aperture optical systems and which could maintain phasing of the multiple apertures would constitute a significant advance in the art.